Recently, X-ray CT apparatus have been able to collect data in a broad range in a short time because of a multi-row structure of X-ray detectors and increase of scanner rotational speed. On the other hand, the exposed dose per examination has increased and thus importance has been put to reduction of X-ray exposure due to a dose-reduced imaging operation. However, simple low-dose imaging causes increase of image noise and thus an image suitable for diagnosis cannot be obtained in some cases.
Therefore, there has been proposed a technique that an image to be obtained under any imaging condition specified by an operator is simulated before the image is picked up as disclosed in Patent Document 1. According to this technique, an image to be obtained under the specified imaging condition can be presented to the operator before the image is picked up, and thus the operator can determine whether an image suitable for diagnosis can be obtained under the specified imaging condition.    Patent Document 1: JP-A-2004-057831    Patent Document 2: JP-A-2006-116137
However, the technique disclosed in Patent Document 1 gives no consideration to what a lesion assumed by an operator looks like on a simulation image, that is, execution of a simulation based on the assumption of occurrence, metastasis or the like of tumor.